1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) communication system, and in particular, to an apparatus and method for canceling Inter-Symbol Interference (ISI) caused by a multipath channel.
2. Description of the Related Art
In recent years, studies have been actively conducted to provide a variety of services to users at high rates (at or above about 100 Mbps) in a 4th generation (4G) system called a future-generation wireless communication system. Particularly, many techniques are required for the physical layer or its upper layers to provide high-speed service. The physical layer adopts multiplexing as a technology for dividing one circuit (a pair of a transmitter and a receiver in wireless communications) and establishing a plurality of communication paths (hereinafter, referred to as “channels”) to enable simultaneous transmission/reception of independent signals, for high-speed data transmission. Classical multiplexing schemes are Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). Particularly, a case of FDM for high-speed data transmission, OFDM was approved and has been studied for use in multiplexing in the physical layer in the 4G wireless communication system under implementation.
OFDM is a special case of Multi-Carrier Modulation (MCM) in which prior to transmission a serial symbol sequence is converted to parallel symbol sequences and modulated to mutually orthogonal subcarriers.
The first MCM systems appeared in the late 1950's for military High Frequency (HF) radio communication, and OFDM with overlapping orthogonal subcarriers was initially developed in the 1970's. In view of difficulty of maintaining orthogonal modulation between multiple carriers, OFDM has limitations in applications to real systems.
However, in 1971, Weinstein, et al. proposed an OFDM scheme that applies Discrete Fourier Transform (DFT) to parallel data transmission as an efficient modulation/demodulation process, which was a driving force behind the development of OFDM. Although hardware complexity was an obstacle to the widespread use of OFDM, recent advances in digital signal processing technology including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) have enabled OFDM implementation. Also, the introduction of a guard interval and a cyclic prefix (CP) as a specific guard interval further mitigated adverse effects of multipath propagation and delay spread.
Due to its feasibility for high-speed data transmission, OFDM was adopted as a standard for fast Wireless Local Area Networks (WLAN) based on the IEEE 802.11a and HIPERLAN/2, IEEE 802.16 Broadband Wireless Access (BWA), and Digital Audio Broadcasting (DAB) standards in the field of wireless communications, and as a standard for Asymmetric Digital Subscriber line (ADSL) and Very high-data rate Digital Subscriber Line (VDSL).
FIG. 1 illustrates the principle of OFDM using a plurality of subcarriers.
If a signal is transmitted on a radio channel without dividing a carrier with a broad bandwidth into carriers with smaller bandwidths, it has a high bit error probability due to multipath fading and Doppler spread. This problem is solved at the cost of very high receiver complexity and difficulty in receiver implementation. Therefore, the carrier is divided into smaller frequency bands f1, f2, . . . , fN, as indicated by reference numeral 101. In this case, highly accurate band-pass filters are required in the frequency domain to distinguish the bandwidths of adjacent carriers from each other. Moreover, a plurality of oscillators are used to generate the carriers. Hence, this method is not viable for real systems.
Reference numeral 102 denotes the frequency spectral characteristics of an OFDM signal. Although a plurality of subcarriers transports data in a manner similar to the signal 101, neither band-pass filters for band separation nor oscillators for generating subcarrier frequencies are required. As described before, the generation of the subcarriers by IFFT in the baseband of a transmitter brings the same effects as the carrier generation for the signal 101, and FFT based on the orthogonality between subcarriers at a receiver facilitates separation of subcarriers from one another without device complexity. One thing to note regarding OFDM is that an interference signal from an adjacent subcarrier should have of a near-zero value at points f1, f2, . . . , fN in the OFDM signal 102, for reliable demodulation.
Reference numeral 103 denotes a time-domain discrete signal created by IFFT-processing the frequency-domain OFDM signal 102. The discrete signal is expressed as Equation (1):
                              x          ⁡                      [            n            ]                          =                              ∑                          k              =              l                        N                    ⁢                                          ⁢                                    X              ⁡                              [                k                ]                                      ⁢                                                  ⁢            exp            ⁢                                                  ⁢                          (                                                -                  2                                ⁢                j                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                k                ⁢                                  n                  N                                            )                                                          (        1        )            where x[n] denotes the time-domain OFDM discrete complex signal having n time-domain samples after IFFT. The variable n ranges from 1 to N. X[k] denotes a frequency-domain discrete complex signal fed as an IFFT input and k is an index indicating the number of the discrete complex signal. N denotes the total number of subcarriers or the number of the time-domain samples, exp( ) denotes an exponential function, and j in the bracket is a complex number. The time-domain signal x[n] may assume an unexpected waveform depending on the amplitude of the transmission data X[k] multiplied by the subcarriers, as indicated by reference numeral 103, and it may have a very large amplitude.
FIG. 2 is a block diagram of a transmitter and a receiver in a typical OFDM wireless communication system.
Referring to FIG. 2, the transmitter includes an encoder 202, a modulator 203, a serial-to-parallel (S/P) converter 204, an IFFT processor 205, a parallel-to-serial (P/S) converter 206, a Cyclic Prefix (CP) inserter 207, and a Radio Frequency (RF) processor 208. The receiver includes an RF processor 210, a CP remover 211, an S/P converter 212, an FFT processor 213, an equalizer 214, a P/S converter 215, a demodulator 216, and a decoder 217.
For transmission from the transmitter, the encoder 202 channel-encodes input information bits at a coding rate to render a transmission signal to be robust on a radio channel. The modulator 203 modulates the coded data according to a modulation scheme. The modulation scheme can be Quadrature Phase Shift Keying (QPSK), 16-ary Quadrature Amplitude Modulation (16QAM), or 64QAM. The S/P converter 204 parallelizes the serial modulation symbol sequence.
The IFFT processor 205 IFFT-processes the parallel data to time-domain sample data. The P/S converter 206 serializes the parallel IFFT data. The CP inserter 207 inserts a CP into the parallel sample data to eliminate ISI caused by the multipath fading of the radio channel. While at first, null data was inserted for a initial period of time as a guard interval, a cyclic prefix or cyclic postfix is now used as the guard interval. The cyclic prefix is formed by inserting a copy of some last bits of a time-domain OFDM symbol before the start of the valid OFDM symbol and the cyclic postfix is formed by inserting a copy of some first bits of a time-domain OFDM symbol after the end of the valid OFDM symbol. The data stream with the guard interval is an OFDM symbol transmitted on a radio channel. Herein, it is assumed that the cyclic prefix is used as the guard interval.
The RF processor 208 converts the digital signal received from the CP inserter 207 to an analog signal, upconverts the analog baseband signal to a transmittable RF signal, and transmits the RF signal on a radio channel 209. During the transmission, the signal experiences multipath fading, a physical phenomenon inherent to the spatial and frequency nature of the radio channel. The resulting ISI is not eliminated by the inserted CP alone.
For reception at the receiver, the RF processor 210 downconverts an RF signal that has experienced the radio channel 209 to a baseband signal and converts the analog baseband signal to time-domain sample data. The CP remover 211 removes the CP from the sample data and outputs a valid OFDM symbol. The S/P converter 212 parallelizes the serial data received from the CP remover 211. The FFT processor 213 FFT-processes the parallel data to frequency-domain data.
The equalizer 214 compensates the FFT data for noise generated on the radio channel 209. The P/S converter 215 converts the equalized parallel data to serial data. The demodulator 216 demodulates the serial data according to a demodulation scheme. The decoder 217 channel-decodes the demodulated data at a coding rate, thereby recovering information data.
The structure of the OFDM symbol will be described in more detail. FIG. 3 illustrates a typical OFDM symbol in the time-frequency domain.
Referring to FIG. 3, the OFDM symbol is divided into a guard interval and a data interval. The data interval is filled with valid data after IFFT, and the guard interval has data inserted to protect the OFDM symbol against multipath delay-caused contamination. As stated before, although the original guard interval was zero data, a copy of a last part of an OFDM symbol is now used as the guard interval. The length of the guard interval is a ratio of one OFDM symbol, such as 1/64, 1/32, 1/16, 1/8, or 1/4. A guard interval length is determined by taking into account a maximum multipath delay in an initial system design.
The OFDM symbol is represented in the frequency domain as a plurality of frequency components f1, f2, . . , fN. Each of the frequency signals has a guard interval being a copy as its last part. These guard intervals form the CP of the OFDM symbol. Some of the frequency components f1, f2, . . . , fN are simply added to the start of the OFDM symbol, and other frequency components are not added. In other words, no signal distortion occurs in the frequency domain. Instead of the CP, the use of zeroes or any other signal as the guard interval adds different frequency components to the OFDM symbol in the frequency domain. As a result, the orthogonality between the subcarriers may be impaired. Past studies already revealed this advantage of the CP.
FIG. 4 illustrates multipath delay-caused data contamination of a downlink OFDM symbol in the typical OFDM communication system. In general, a downlink OFDM symbol arrives at a downlink receiver from different paths. Two major paths 410 and 420 that most significantly affect data recovery are shown.
Referring to FIG. 4, the OFDM symbol is divided into a CP and data. The CP and a last part of the data are labeled with the same numeral because the CP is a copy of the last data part. In view of the multipath delay, an OFDM symbol 411 arrives at the receiver in the first path 410 earlier than an OFDM symbol 422 in the second path 420 by a time delay 421. A second symbol 414 in the first path 410 overlaps with the first symbol 422 in the second path 420. This overlap acts as so-called ISI to the second symbol 414.
On the part of the second symbol 414, the overlap with the first symbol 422 in the second path is a delay spread 423. If the delay spread 423 is shorter than a CP 412 of the OFDM symbol 414, it does not affect data recovery gain. On the other hand, if the delay spread 423 is longer than the CP 412, data contamination 413 occurs, functioning as noise in data recovery of the second symbol 414. As a result, a data reception gain is usually decreased.
Although the data contamination is prevented by lengthening the CP, the increased CP length makes symbol synchronization difficult and decreases frequency efficiency. Most of methods proposed so far use an additional device or algorithm for eliminating data contamination components in a receiver, or channel coding designed to be very robust against contamination in a transmitter. These conventional methods commonly suffer from increased receiver complexity and decreased frequency efficiency.
FIG. 5 illustrates multipath delay-caused data contamination of an uplink OFDM symbol in the typical OFDM communication system.
Referring to FIG. 5, timing synchronization is acquired between a Base Station (BS) and Subscriber Stations (SS) to enable propagation of successive OFDM symbols in the first path on the uplink. However, the successive transmission of OFDM symbols is not ensured for the second path. To be more specific, an OFDM symbol 513 from a user 512 (user #1) is received at the BS, followed by an OFDM symbol 517 from a user 516 (user #2) in a first path 510. However, an OFDM symbol 522 from user #1 and an OFDM symbol 524 from user #2 may arrive at the BS with different time delays from a second path 520. For example, the OFDM symbol 522 from user #1 is received at the BS from the second path 520 with a long time delay 521, whereas the OFDM symbol 524 from user #2 at a different location within the same cell is received at the BS with a short time delay 523 from the second path 520.
Due to the time delay 521 in the second path 520, the data from user #1 suffers from phase discontinuity 511 and thus has a decreased bit error performance. The data from user #2 is subject to contamination due to the delay spread resulting from the time delay 521.